1. Field of the Invention
The present invention relates to a tunable laser which can vary the oscillation wavelength thereof.
2. Description of the Related Art
As broadband communications are becoming more and more popular, attempts are being made to introduce WDM (Wavelength Division Multiplexing) transmission systems that are capable of communications at a plurality of different optical wavelengths over a single optical fiber for efficient utilization of fiber optic transmission paths. Recently, DWDM (Dense WSD) apparatus for multiplexing optical signals of several tens of wavelengths for more rapid transmission have also been finding growing use. Efforts have also been made to commercialize ROADM (Reconfigurable Optical Add/Drop Multiplexers) for adding and dropping optical signals of desired wavelengths at each node in the optical networks. If ROADM systems are introduced into the communication environment, then the flexibility of optical networks will dramatically be increased because they are capable of optical path switching by changing wavelengths as well as the transmission capacity is increased by wavelength multiplexing.
The WDM transmission system requires as many light sources as the number of wavelengths that are employed. Therefore, as the number of wavelengths to be multiplexed by the WDM transmission system increases, the number of light sources that are required also increases.
DFB-LDs (Distributed FeedBack Laser Diodes) which oscillate in a single axial mode have widely been used as light sources in WDM transmission systems for the ease and reliability with which they can be used. The DFB-LD has a diffraction grating having a depth of about 30 nm which is disposed entirely in the resonator. The DFB-LD oscillates stably in the single axial mode at a wavelength corresponding to the product of the period of the diffraction grating and a value that is twice the equivalent refractive index. However, since it is impossible to tune the DFB-LD for a wide range of oscillation wavelengths, a DFB-LD-based WDM transmission system employs DFB-LD devices having different wavelengths for respective ITU (International Telecommunication Union) grids. The need for using DFB-LD devices having different wavelengths makes the DFB-LD-based WDM transmission system problematic because the shelf control cost is high and a redundant inventory of DFB-LDs is required in preparation for DFB-LD failures. If the ROADM system that is capable of optical path switching by changing wavelengths employs ordinary DFB-LDs, then the variable extent of the wavelength range is limited to about 3 nm that can be changed with a temperature change, making it difficult to construct an optical network incorporating the features of the ROADM that positively uses wavelength resources.
Intensive research has been conducted on tunable lasers in order to solve the problems of the present DFB-LDs and achieve single-axial-mode oscillation in a wide range of wavelengths. One example of such research efforts is shown in “Hikari Syuseki Devices (Optical integrated devices)”, written by Isao Kobayashi, first edition, second printing, Kyoritsu Shuppan Co., Ltd., December 2000, pages 104-122. Some examples described in this literature will be given below to describe conventional tunable lasers.
Tunable lasers are generally classified into two types, i.e., tunable lasers with a wavelength varying mechanism disposed in a laser element and tunable lasers with a wavelength varying mechanism disposed outside of a laser element.
One proposed tunable laser with a wavelength varying mechanism disposed in a laser element is a DBR-LD (Distributed Bragg Reflector Laser Diode) having an active region for producing a gain and a DBR region for producing a reflection with a diffraction grating, the active region and the DBR region being disposed in one laser element. The DBR-LD has a variable wavelength range of about 10 nm at maximum. There has also been proposed a DBR-LD employing a nonuniform diffraction grating which has an active region for producing a gain and front and rear DBR regions sandwiching the active region. The active region and the DBR regions are disposed in one laser element. In the front and rear DBR regions, the nonuniform diffraction grating produces a number of reflection peaks spaced at intervals that are slightly different in the front and rear DBR regions. Since this structure causes a vernier effect, the DBR-LD with the nonuniform diffraction grating makes it possible to change wavelengths in a very wide range, and can achieve wavelength varying operation in a range in excess of 100 nm and can achieve quasi-continuous wavelength varying operation in a range of 40 nm.
One proposed tunable laser with a wavelength varying mechanism disposed outside of a laser element is a tunable laser having a diffraction grating disposed outside of a laser element, the diffraction grating being rotatable to return light at a particular wavelength to the laser element. The tunable laser of this type requires a mechanism for sequentially monitoring oscillating wavelengths. Heretofore, a wavelength-selective component such as an etalon or the like is incorporated in the module for monitoring oscillating wavelengths.
Though many structures have been proposed for use as conventional tunable lasers, it has been difficult to put them to practical use because of various problems including mode hopping, complex wavelength control, weak vibration resistance, and high cost due to device enlargement.
The DBR-LD changes wavelengths by injecting carries into the DBR region to change the refractive index thereof. If crystal defects grow on account of the electric current injection, then the ratio of a refractive index change to the electric current injection changes greatly, making it difficult to maintain laser oscillation at a constant wavelength over a long period of time. Since the DBR-LD is of a complex structure, it tends to have a large size. According to the present compound semiconductor device fabrication process technology, it is impossible to increase the size of a laser substrate by 2 inches (50.8 mm) or more. Consequently, it is difficult to reduce the present price of DBR-LDs.
The tunable lasers with the wavelength varying mechanism disposed in the laser element are liable to bring about mode jumping due to vibration. These tunable lasers need a large vibration-resistant mechanism and tend to have a large module size and an increased cost. The tunable lasers also suffer an increased assembling cost as they require many optical components such as a photodetector in addition to the etalon for monitoring oscillating wavelengths. It has been customary to spatially couple the laser emitting surface and the etalon to each other with a lens for wavelength monitoring. According to the customary approach, a slight positional error of the etalon is apt to vary the accuracy of wavelengths. Therefore, the highly accurate mounting technology is required to install the etalon in position. However, the highly accurate mounting technology is also responsible for an increase in the assembling cost of tunable lasers.